How To Graft Tomatoes (GTF Method)

Gathering Together Farm has been growing a significant portion of its tomato crop in greenhouses for over 15 years. Because of the limited space available under cover, the greenhouse rotation schedule has been less than ideal, making it necessary to plant tomatoes in the same houses more frequently than recommended. Over the years, the soil in particular greenhouses began to harbor various diseases that were stunting the growth and production of the tomato plants grown under cover. It became apparent that something needed to change if the farm was going to continue to plant tomatoes in existing greenhouses. The solution was grafting. Now, about 70-80% of the Gathering Together Farm tomato crop comes off grafted plants.

In essence, grafting tomatoes lets farmers reap the yield off of the tomato variety of their choice, but that scion is grown on highly disease resistant rootstocks. The result is more vigorous plants, a larger yield of higher quality fruits, and a longer period of productivity. Tomato grafting has been practiced for decades by growers who have been planting tomatoes in the same ground year after year because of limited space for necessary crop rotation.

At Gathering Together Farm, Paula has been grafting tomatoes for four years, and at this point, she has a fairly high success rate, about 90-95%. Paula and Joelene learned the art of grafting tomatoes from reading the literature online and in print, experimenting, and collaborating with Alice at Log House Plants, who was figuring out grafting methods around the same time.

To start off, ‘Maxifort‘ rootstock from Johnny’s Selected Seeds are seeded into 2.5″ pots, and all the other varieties of tomato scion are seeded into flats with 98 cells (“100s”). Even though the seed is relatively expensive, Gathering Together Farm over-seeds the rootstock by about 40% to account for an 80% germination rate and the fact that some rootstock plants will be unsuitable for grafting. Trays of seeded pots and flats are placed into a dark, humid chamber heated to about 85°.

The sprouting chamber is a farm-made structure with grated shelving underlain with rubber hoses through which warm water flows from an electric water heater. The sides and doors are plastic with velcro closures.

Starting four days after seeding, Joelene pulls every tray of pots seeded with rootstock out of the hot chamber and examines each pot for any signs of above-ground sprouting. If even the slightest bit of white tendril is showing, she pulls the pot and joins it with other sprouted tomatoes on a warmed table in the greenhouse. She sorts the new tomato seedlings daily until it becomes apparent than anything left is never going to sprout (up to two weeks after seeding). Grouping seedlings in like-staged trays will allow Paula to more easily match rootstock and scion at the exact same stem size.

The scion varieties generally sprout at about the same time, so Joelene pulls them out of the sprouting chamber when she sees any above-ground growth.

Tomato starts grow on heated tables for about three weeks.

There’s some debate among the grafting team about whether or not the rootstock plants should be placed under lights after seedlings are pulled from the sprouting chamber. At this time, it is believed that the lights may stunt the elongation of the stems, so the rootstock plants are allowed to grow on warmed tables without lights while scion plants do grow under lights.

It is critical when grafting for the stems of the rootstock plants and the stems of the scion plants to be the same diameter. If it seems like the rootstock or the scion is outpacing the other’s growth, Paula will move trays off the warming tables to slow growth. Sometimes she will even move trays to the ground in the shade if she needs to let the plant parts match catch up.

Here’s what the ‘Maxifort’ rootstock looks like when it’s ready for grafting. Ideally, the rootstock plants will have straight stems, but sometimes they don’t. Some bent-stemmed rootstock plants can be used for grafting, but really crooked ones or plants with the cotyledons too close to the soil surface are discarded.

The day before she plans to graft, Paula selects rootstock plants that will match up with the scion.

She waters the rootstock well because they can’t be watered again directly for a couple weeks.

She puts the rootstock under lights in a last ditch effort to encourage the stems to grow as straight as possible.

Paula moves the scion into a dark area to slow photosynthesis so that the plants are as dormant as possible when grafting.

For grafting, Paula splits these double edge razor blades (from Fred Meyer) and uses one blade for two trays of grafts (64 plants) before discarding it.

First, she pinches the top of the rootstock and swiftly cuts the stem at a 45° angle. She must cut the stem at least an inch above the soil level, so that when the tomato start is transplanted into the ground, the scion stem will not touch the earth. She also cuts the stem below the cotyledons, so that the rootstock will not produce any foliage or fruit.

She discards the rootstock top.

Then she finds a matching scion plant and cuts its stem at a 45° angle.

This is a tray of mostly cut scion.

She fits the clip on the rootstock stem and then slides the scion in, matching up the 45° angles.

The line of symmetry of the clip should be perpendicular to the plane of the graft so that the clip is in contact with the rootstock and the scion equally.

If she’s grafting larger seedlings, Paula will sometimes cut off the cotelydons to decrease the weight of the scion and keep it from slumping over and putting extra pressure on the graft.

She mists the inside of the cover, tightly closes the top vents, and fits it tightly on the bottom tray, eliminating any airflow and preventing the plants from drying out. Gathering Together Farm purchased the covers from McConkey.

The post-graft healing “building” is a structure built inside the Gathering Together Farm propagation greenhouse. It’s divided into two chambers, a dark chamber and a shaded chamber. Each chamber has built-in shelving for trays of grafted tomatoes with lids. The dark chamber, is covered with black plastic under a reflective tarp (silver side out to prevent the absorption of heat). The shaded chamber is covered with white plastic under shade cloth with some insulation on the south side to keep it from getting too hot. The healing building offers a shelter with limited temperature fluctuations, limited sunlight (or no sunlight), and limited airflow that will allow the plants to heal slowly without attempting to photosynthesize until the graft is set.

The trays of newly grafted plants are placed on shelves in the dark chamber and left there for about three days.

After three days in the dark, tomato plants are moved onto shelves in the shaded chamber for a day, and then they stay in the same chamber with overhead lights for another day.

After a day under lights, the top vents on the cover are opened.

The next day, the covers are propped open to allow limited air to flow through the trays. A day or so later, the covers are taken off completely.

Eventually, the uncovered trays are moved out to unheated tables in the propagation greenhouse.

With each step after the trays come out of the dark, the plants are assessed, and if they seem to be wilting or looking sickly, Paula will move them back a step and wait another day to try again.

About a month after grafting, the joint between rootstock and scion will have healed, and the stems will have grown enough so that the clips will start to pop off or they can be removed by hand.

Grafted tomato plants will be transplanted into the ground a few weeks later. When planting, it is critical that the graft line remain well above ground, so that the scion will not root into the soil. The graft will remain visible for the full life of the plant.

This is the Gathering Together Farm tomato-grafting method for the 2012 season It is probably different than any of the tomato-grafting methods practiced around the world. As we continue to learn from our mistakes and experiment with new techniques, this process may change in little or perhaps big ways.